Wide input voltage range power supply with auto-transformers and piezoelectric transformer

ABSTRACT

N-channel enhancement type switching transistors complementarily turn on and off in response to pulse signals complementary to each other so as to cause two auto-transformers to alternately supply secondary potentials to primary electrodes of a piezoelectric transformer, and the piezoelectric transformer supplies an alternating current signal to a load; the primary winding portion of each auto-transformer is dually used in the accumulation of current energy and the resonating phenomenon for generating the secondary potential, and, for this reason, the auto-transformers make the driving circuit for the load compact.

FIELD OF THE INVENTION

This invention relates to a driving circuit and, more particularly, to asmall thin driving circuit with auto-transformers and a piezoelectrictransformer operable under a wide power voltage range.

DESCRIPTION OF THE RELATED ART

A piezoelectric transformer is a voltage-to-voltage converter on thebasis of a piezoelectric effect, and has primary and secondaryelectrodes differently polarized on a piezoelectric element. When apotential level is applied to the primary electrode, the piezoelectrictransformer converts the potential level to a different potential levelthrough the piezoelectric effect. The piezoelectric transformer iseasily scaled down rather than an electromagnetic transformer, and isdesirable for a power source of a back light incorporated in a liquidcrystal display unit.

A typical example of the driving circuit for the piezoelectric elementis disclosed in Japanese Patent Publication of Unexamined ApplicationNo. 3-139178, and FIG. 1 illustrates the prior art driving circuit. FIG.2 shows an equivalent circuit of the prior art driving circuit. Theprior art driving circuit is associated with a piezoelectric element 1or an ultrasonic motor, and comprises a 2-phase pulse generator 2, twotransformers 3a and 3b and two switching transistors 4a and 4b.

The 2-phase pulse generator 2 generates first and second pulse signalsPL1 and PL2 different in phase from one another, and the first andsecond pulse signals PL1 and PL2 are supplied to the gate electrodes ofthe switching transistors 4a and 4b, respectively.

The switching transistors 4a and 4b are connected between the primarywindings of the transformers 4a/4b and the ground, and the secondarywindings of the transformers 3a/3b are connected to primary electrodesof the piezoelectric element 1.

The switching transistors 4a and 4b alternately turn on and off, andgenerate a resonant voltage waveform in the primary windings. Theresonant voltage waveform is boosted from the primary windings to thesecondary windings, and the boosted resonant voltage waveform is appliedto the piezoelectric element 1.

FIG. 3 illustrates the circuit behavior of the prior art drivingcircuit. The switching transistor 4a turns on at time t1, and turns offat time t2. The switching transistor 4a gives rise to an increase of thedrain current Id1 between time t1 and time t2, and the drain voltage Vd1is approximately equal to the ground level therebetween. The draincurrent Id1 is expressed by equation 1.

    Id1=VDD×t/Lp1                                        Equation 1

where VDD is the positive power voltage applied to the primary windings,Lp1 is the inductance of on the primary winding of the transformer 3aand t is time. The drain current Id1 forms a saw tooth pulse.

The switching transistor 4a is turned off between time t2 and time t3.When the switching transistor 4a turns off, the electromagnetictransformer 3a releases the accumulated current energy as voltageenergy, and boosts the voltage at the turn ratio so as to apply it tothe piezoelectric element 1. The resonance takes place due to the inputcapacitance C of the piezoelectric element 1 (see FIG. 2) and theinductance on the secondary winding Ls1, and forms the half of asinusoidal wave.

On the other hand, the other switching transistor 4b turns off at timet1, and turns on at time t2. The switching transistor 4b is turned onfrom time t2 to time t3. Thus, the switching transistors 4a and 4bcomplementarily turns on and turns off, and generates the resonantvoltage waveform Vout between the secondary windings. The resonantvoltage waveform is equivalent to a sinusoidal wave at the piezoelectricelement 1, and, for this reason, the vibrations on the piezoelectrictransformer 1 contain small amount of harmonics.

Although the prior art driving circuit shown in FIG. 1 aims atgeneration of the mechanical vibrations, the prior art driving circuitis available for a piezoelectric transformer, and is expected to stablycontrol current/voltage applied to the load. Moreover, the prior artdriving circuit is expected to drive the piezoelectric transformeraround a resonant frequency determined by the physical configurationthereof and further to be integrated with the piezoelectric transformerwithout enlargement. Thus, the prior art driving circuit for thepiezoelectric transformer is different in technical goal from thedriving circuit for the piezoelectric element 1.

As described hereinbefore, while the prior art driving circuit isdriving the piezoelectric element 1, the drain current Id1 flows asexpressed by equation 1. The other electromagnetic transformer 3b causesthe drain current Id2 to complementarily flow through the switchingtransistor 4b, and the amount of the drain current Id2 is similarlyexpressed. When the power voltage VDD is increased, the electromagnetictransformers 3a/3b proportionally increase the drain currents Id1/Id2,and a designer needs to set a limit on the maximum drain current,because the magnetic saturation causes the electromagnetic transformers3a/3b to lose the inductance. The loss of the inductance results inlarge amount of current and, accordingly, damages of the electromagnetictransformers 3a/3b and the associated switching transistor 4a/4b. Thedesigner gives a large margin to the maximum drain current, and thelarge margin enlarges the electromagnetic transformers 3a/3b.

When the power voltage VDD is widely changed, a prior art power sourceor a prior art inverter limits the peak current by decreasing theswitching intervals. However, in the case where the prior art ultrasonicmotor driving circuit is used for a piezoelectric element, the drivingfrequency is allowed to vary within several percent, because thepiezoelectric element has a large quality factor and a narrow resonantfrequency range. This means that the prior art driving circuit hardlycopes with the wide variation of the power voltage VDD. If thefluctuation in the power voltage VDD is wide, the driving voltage isproportionally increased, and the large amount of driving voltagedestroys the piezoelectric element. Thus, the stabilization of the powervoltage is requested for the prior art driving circuit.

If a dc-to-dc converter is connected to the prior art driving circuit,the dc-to-dc converter makes the power voltage VDD stable. However, thedc-to-dc converter deteriorates the efficiency of the prior art drivingcircuit, and cancels the high efficiency of the piezoelectric element.Although the dc-to-dc converter coupled to the driving circuit makes thepower voltage stable, the dc-to-dc converter decreases the efficiency ofthe piezo-electric element. The dc-to-dc converter is less appropriatebecause of its complicated arrangement.

The prior art driving circuit requires large switching transistors 4a/4bso as to discharge the drain currents Id1/Id2, and the large switchingtransistors 4a/4b make the prior art driving circuit thick. Thus, aproblem is encountered in the prior art driving circuit in the largevolume due to the large switching transistors 4a/4b and theelectromagnetic transformers 3a/3b.

In summary, the prior art driving circuit encounters a problem in largevolume. Moreover, the prior art driving circuit further has problems inthe narrow driving frequency range and in the narrow power voltagerange.

SUMMARY OF THE INVENTION

It is therefore an important object of the present invention to providea driving circuit which is small, thin and operable under a wide voltagerange.

To accomplish the object, the present invention proposes to useauto-transformers.

In accordance with the present invention, there is provided a drivingcircuit for a load comprising: a first auto-transformer including afirst winding having a first node splitting the first winding into afirst primary winding portion connected to a power source and a firstsecondary winding portion; a second auto-transformer including a secondwinding having a second node splitting the second winding into a secondprimary winding portion connected to the power source and a secondsecondary winding portion; a first switching transistor including afirst control node and a first current path connected between the firstnode and a constant voltage source; a second switching transistorincluding a second control node and a second current path connectedbetween the second node and the constant voltage source; a piezoelectrictransformer including primary electrodes respectively connected to thefirst secondary winding portion and the second secondary windingportion, and a secondary electrode connected to the load; and a pulsegenerating means for supplying a first pulse signal and a second pulsesignal complementary to the first pulse signal to the first control nodeand the second control node, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the driving circuit according to thepresent invention will be more clearly understood from the followingdescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a circuit diagram showing the prior art driving circuit;

FIG. 2 is a circuit diagram showing the equivalent circuit of the priorart driving circuit;

FIG. 3 is a graph showing the drain voltage, the drain current and theresonant voltage waveform in terms of time;

FIG. 4 is a circuit diagram showing a driving circuit according to thepresent invention;

FIG. 5 is a circuit diagram showing the equivalent circuit of a boostingcircuit incorporated in the driving circuit;

FIGS. 6A to 6C are circuit diagrams showing a standard electromagnetictransformer, an equivalent electromagnetic transformer and anauto-transformer incorporated in the driving circuit;

FIG. 7 is a block diagram showing the circuit arrangement of a frequencycontrolling circuit incorporated in the driving circuit;

FIG. 8 is a graph showing current-to-frequency characteristics of thefrequency controlling circuit;

FIG. 9 is a graph showing potential levels at essential nodes ofelectromagnetic transformers incorporated in the driving circuit;

FIG. 10 is a graph showing potential levels at essential nodes of apiezoelectric transformer incorporated in the driving circuit;

FIG. 11 is a graph showing currents flowing through the essential nodesof the electromagnetic transformer;

FIG. 12 is a circuit diagram showing the equivalent circuit of theelectromagnetic transformers and the piezoelectric transformer;

FIGS. 13A to 13C are views showing the waveforms of currents flowingthrough the electromagnetic transformers; and

FIG. 14 is a graph showing the currents under fluctuation of a powervoltage.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 4 of the drawings, a driving circuit 11embodying the present invention is connected to a load 12, and largelycomprises a boosting circuit 11a, a piezoelectric transformer 11b and afrequency controlling circuit 11c.

The boosting circuit 11a includes a 2-phase pulse generator 11d forgenerating first and second pulse signals PL11 and PL12, and the firstpulse signal PL11 is complementary to the second pulse signal PL12. Thefrequency controlling circuit 11c supplies a clock signal CLK1 to the2-phase pulse generator 11d, and the 2-phase pulse generator 11dgenerates the first and second pulse signals PL11 and PL12 from theclock signal CLK. As will be described hereinafter, the clock signal CLKis variable in frequency, and, accordingly, changes the frequency of thefirst and second pulse signals PL11/PL12. In this instance, thefrequency controlling circuit 11c and the 2-phase pulse generator 11d asa whole constitute a pulse generating means.

The boosting circuit 11a further includes first and secondelectromagnetic transformers 11e and 11f and first and second n-channelenhancement type switching transistors 11g and 11h. The first pulsesignal PL11 and the second pulse signal PL12 are respectively suppliedto the gate electrode of the n-channel enhancement type switchingtransistor 11g and the gate electrode of the n-channel enhancement typeswitching transistor 11h, and the first and second n-channel enhancementtype switching transistors 11g/11h provide current paths betweenintermediate nodes of the first and second electromagnetic transformers11e/11f and a ground line GND.

The first and second electromagnetic transformers 11e and 11f areconnected between a power voltage line VDD and primary electrodes of thepiezoelectric transformer 11b, and a secondary electrode of thepiezoelectric transformer 11b is connected to the load 12.

The piezoelectric transformer 11b supplies a sinusoidal resonant drivingsignal Vin to the load 12, and a discharge current Iout flows from theload 12 to the frequency controlling circuit 11c.

If the load 12 is a cold cathode fluorescent lamp, the driving circuit11a is equivalent to a circuit configuration shown in FIG. 5. In thisinstance, the output current Is2 of the electromagnetic transformer 11fflows through the primary winding of an equivalent transformer TF, anequivalent resistor R, an equivalent capacitor C, an equivalent inductorL and an input capacitor Cd1 into the secondary winding of theelectromagnetic transformer 11e. The electromagnetic transformers 11eand 11f are auto-transformers. FIGS. 6A to 6C illustrate a standardelectromagnetic transformer, an equivalent electromagnetic transformerand an auto-transformer, and the turn ratio of the transformers 13a, 13band 13c is 1:N.

FIG. 6A shows the transformer 13a having the primary winding connectedbetween an alternating current source 14 and the ground line and thesecondary winding connected to a load RL and the ground line. When thealternating current source 14 supplies an alternating voltage E Vp-p!,the secondary winding generates an alternating voltage NE Vp-p!. If thesecond winding is connected to the alternating current source 14 asshown in FIG. 6B, the alternating voltage N Vp-p! is added to the outputvoltage of the secondary winding, and the output voltage is expressed as(N+1)E Vp-p!.

If the alternating current source 14 is connected to an intermediatenode 13d so as to split a winding 13e at N:1, the transformer 13c servesas the auto-transformer equivalent to the transformer 13b, and appliesthe output voltage (N+1)E Vp-p! to the load RL. Each of theelectromagnetic transformers 11e/11f is similar in configuration to theauto-transformer 13c, and is advantageous over the transformer 13b inlarge step-up ratio under the same turns or a smaller number of turnsunder the same step-up ratio.

The electromagnetic transformers 11e and 11f are similar to theauto-transformer, and make the boosting circuit 11a compact.

FIG. 7 illustrates the circuit arrangement of the frequency controllingcircuit 11c. The frequency controlling circuit 11c includes acurrent-to-voltage converter 11k, a rectifier 11m and comparator 11n.The current-to-voltage converter 11k is, by way of example, implementedby a resistor. The current-to-voltage converter 11k converts thedischarge current Iout to an alternating voltage signal Vout1, and therectifier 11m generates a dc voltage signal Vdc from the alternatingvoltage signal Vout1. The comparator 11n compares the dc voltage signalVdc with a first reference voltage signal Vref1. When the dc voltagesignal Vdc is lower than the first reference voltage signal Vref1, thecomparator 11n generates a high level signal. On the other hand, if thedc voltage signal Vdc is higher than the first reference voltage signalVref1, the comparator 11n generates a low level signal.

The frequency controlling circuit 11c further includes an integrator11o, a voltage-controlled oscillator 11p abbreviated as "VCO" and acomparator 11q. While the comparator 11n is supplying the high levelsignal to the integrator 11o, the integrator 11o decreases a frequencycontrol signal Vout2 at a predetermined rate, and the frequency controlsignal Vout2 is supplied to the voltage-controlled oscillator 11p. Thevoltage controlled oscillator 11p is responsive to the frequency controlsignal Vout2 so as to change the frequency of the first and second pulsesignals PL11/PL12. On the other hand, if the comparator 11n supplies thelow level signal to the integrator 11o, the integrator 11o maintains thefrequency control signal Vout2 at a potential level immediately beforethe change from the high level signal to the low level signal.

The comparator 11q compares the frequency control signal Vout2 with asecond reference voltage signal Vref2, and supplies a reset signal RSTto the integrator 11o as follows. If the power voltage VDD is decreasedor the load 12, such as a cold cathode fluorescent lamp, does not supplythe discharge current Iout to the current-to-voltage converter 11k untillamp activation, the comparator 11n continuously supplies the high levelsignal, and the integrator 11o causes the voltage-controlled oscillator11p to decrease the frequency of the first and second pulse signalsP11/PL12 from an initial value f1 to the minimum value f2 (see FIG. 8).Then, the frequency control signal Vout2 becomes lower than the secondreference voltage signal Vref2, and the comparator 11q generates thereset signal RST. The integrator 11o causes the frequency control signalVout2 to return to an initial value corresponding to the initialfrequency f1. Thus, the comparator 11q and the integrator 11o cause thevoltage-controlled oscillator 11p to loop the frequency of the first andsecond pulse signals PL11/PL12 between f1 and f2.

Assuming now that the frequency controlling circuit 11c starts thefrequency regulation, the integrator 11o sets the frequency controlsignal Vout2 to the initial value, and the voltage controlled oscillator11p oscillates so as to generate the clock signal CLK at the initialfrequency f1. The initial frequency f1 is higher than a target frequencyft close to a resonant frequency fr of the piezoelectric transformer11b, and the piezoelectric transformer 11b is controlled at thefrequency f1. In this situation, the dc voltage signal Vdc is lower thanthe first reference voltage signal Vref1, and the comparator 11n causesthe integrator 11o to decrease the frequency control signal Vout2. As aresult, the voltage controlled oscillator 11p decreases the frequency fas indicated by arrow AR. When the discharge current Iout reaches atarget value It, the voltage controlled oscillator regulates thefrequency f to the target value ft.

If the power voltage VDD is too low to obtain the target dischargecurrent It as indicated by plots PLT1, the clock signal CLK reiteratesthe loop between f1 and f2 until a recovery of the power voltage VDD.When the power voltage VDD is recovered to a standard level, thefrequency controlling circuit 11c changes the clock signal CLK alongplots PLT2.

On the other hand, if the power voltage VDD is higher than the standardlevel, the frequency controlling circuit 11c changes the clock signalCLK along plots PLT3, and the target frequency ft' is slightly spacedfrom the resonant frequency fr.

As will be understood from the foregoing description, even if the powervoltage VDD fluctuates, the frequency controlling circuit 11c regulatesthe clock signal CLK and, accordingly, the first and second pulsesignals PL11/PL12 to a neighborhood of the resonant frequency fr.

Description is hereinbelow made on the circuit behavior of the drivingcircuit 11 with reference to FIGS. 9, 10 and 11.

When the driving circuit is powered, the frequency controlling circuit11c firstly supplies the clock signal CLK at the initial frequency f1,and regulates the clock signal CLK to the target frequency ft asdescribed previously. The 2-phase pulse generator 11d respectivelysupplies the first and second clock signals PL11/PL12 to the first andsecond n-channel enhancement type switching transistors 11g/11h, and thefirst and second n-channel enhancement type switching transistors 11gand 11h complementarily turn on and off. In this instance, the firstn-channel enhancement type switching transistor 11g turns on at time t11and t13, and the second n-channel enhancement type switching transistor11h turns on at time t10, t12 and t14.

While the first and second n-channel enhancement type switchingtransistors 11g/11h are turned on, drain currents Id1 and Id2 flows intothe ground line GND, and the power voltage line VDD accumulates currentenergies Ip1/Ip2 into the primary windings of the first and secondelectromagnetic transformers 11e/11f. When the first and secondn-channel enhancement type switching transistors 11g/11h turn off, theaccumulated energies are released, and primary potential levels Vd1 andVd2 take place as shown in FIG. 9. The primary potential levels Vd1/Vd2are about three times higher than the power voltage VDD.

The primary potential levels Vd1 and Vd2 are boosted to secondarypotential levels Vs1 and Vs2 and the secondary potential levels Vs1 andVs2 are (N+1) times higher than the primary potential levels Vd1 andVd2, respectively. The currents Id1, Ip1 and Is1 vary as shown in FIG.11.

FIG. 12 illustrates an equivalent circuit of the essential parts of thedriving circuit 11, i.e., the electromagnetic transformers 11e/11f, thepiezoelectric transformer 11b and the n-channel enhancement typeswitching transistors 11g/11h. The equivalent circuit is viewed from theprimary electrodes 11i of the piezoelectric transformer 11b, and theload 12 is assumed to be a cold cathode fluorescent lamp. The resistanceR and the capacitance C of the cold cathode fluorescent lamp areincorporated in the equivalent ideal transformer of the piezoelectrictransformer 11b. The primary potentials Vd1/Vd2 and the secondarypotentials Vs2/Vs1 form resonant waveforms due to the equivalent inputcapacitance CL of the piezoelectric transformer 11b and the load 12 (seeFIG. 12) and the total inductance of the primary/secondary inductancesLp2/Ls2 or Lp1/Ls1; each resonant waveform is a half of a sinusoidalwave crossing zero volt at a half resonant period of the piezoelectrictransformer 11b. The n-channel enhancement type switching transistor11g/11h in the on-state causes the other electromagnetic transformer11e/11f to have the primary winding coupled to the low impedance powervoltage line VDD and the ground GND and the secondary windingshort-circuited, and the other electromagnetic transformer 11e/11f doesnot have an influence on the resonance.

The electromagnetic transformers 11e and 11f alternately supply the halfsinusoidal waves to the primary electrodes 11i of the piezoelectrictransformer 11b, and the piezoelectric transformer 11b generates thesinusoidal driving signal Vin as shown in FIG. 10. The half sinusoidalwaves are expressed as 6(N+1)VDD Vp-p!. The piezoelectric transformer11b is assumed to boost them at "M", and the potential level of thesinusoidal driving signal Vin is expressed as 6M(N+1)VDD Vp-p!.

The sinusoidal driving signal Vin is supplies to the load 12, and thedischarge current Iout flows from the load 12 to the current-to-voltageconverter 11k of the frequency controlling circuit 11c. The frequencycontrolling circuit 11c regulates the clock signal CLK to the targetfrequency ft as described hereinbefore, and the 2-phase pulse generator11d, the n-channel enhancement type switching transistors 11g/11h andthe electromagnetic transformers 11e/11f cause the piezoelectrictransformer 11b to resonate around the resonant frequency fr.

As shown in FIG. 2, the secondary current flows from one of the primaryelectrodes, and is discharged to the ground. Therefore, the primaryelectrodes are assumed to be independently grounded, and, accordingly,the electromagnetic transformers 3a and 3b are electrically separatedfrom each other.

On the other hand, the driving circuit according to the presentinvention does not electrically separate the electromagnetictransformers 11e and 11f as shown in FIG. 12, and the secondary currentIs1/Is2 flows into the secondary winding of the other electromagnetictransformer 11f/11e as described hereinbelow.

FIGS. 13A, 13B and 13C illustrates the waveforms of the currents Id1,Ip1 and Is2 flowing through the electromagnetic transformers 11e/11f.The n-channel enhancement type switching transistor 11g turns on at timet21 and, accordingly, the other switching transistor 11h complementarilyturns off.

The electromagnetic transformer 11f has been charged before t21, and aresonant potential wave takes place in the primary winding of theelectromagnetic transformer 11f at time t21. The electromagnetictransformer 11f boosts the resonant potential wave, and the current Is2flows through the piezoelectric transformer 11b and the load 12 as shownin FIG. 13C. The current Is2 passes through between the primaryelectrodes of the piezoelectric transformer 11b, and is dischargedthrough the switching transistor 11g to the ground line GND. Theelectrostatic capacitance of the electromagnetic/piezoelectrictransformers 11e/11f/11b is determined in such a manner that the currentIs2 is recovered to zero at time t21.

The primary inductance Lp1 of the electromagnetic transformer 11eincreases the current It proportionally to the lapse of time from theturn-on, and the electromagnetic transformer 11e magnifies the currentIs2 to the current Is2' at "N". As a result, the total Id1 of thecurrents It, Is2 and Is2' forms a non-linear waveform, and flows throughthe switching transistor 11g as shown in FIG. 13A.

The primary winding of the electromagnetic transformer 11e allows thetotal current of It and Is2' to flow therethrough as shown in FIG. 13B.

The n-channel enhancement type switching transistor 11g turns off attime t22, and the other n-channel enhancement type switching transistor11h complementarily turns on. Then, the current Is2 inversely flows. Thecurrent Is2 has negative value, and the current Is1 has positive value.This results in the waveform of the secondary current Is1 or -Is2 shownin FIG. 11.

The primary winding of the electromagnetic transformer 11e decreases theIt and Is2' to zero at time t22 due to the switching action of thetransistor 11g. However, the secondary current Is1 flows through thesecondary windings of the electromagnetic transformer 11e, and thecurrent Ip1 forms the non-linear waveform shown in FIG. 11.

This is the reason why the currents Id1/Id2 non-linearly flows throughthe n-channel enhancement type switching transistors 11g/11h and aredifferent from the saw tooth waveform of the prior art driving circuit.

Subsequently, description is made on the circuit behavior of the drivingcircuit under the fluctuation of the power voltage VDD. When the powervoltage VDD is increased, the drain voltage Vd1 is proportionallyincreased as indicated by arrow AR2 in FIG. 14, and the secondarypotential Vs1 is also proportionally increased. The frequencycontrolling circuit 11c shifts the clock pulse CLK to a higher frequencyso as to restrict the discharge current Iout, and the high frequencyclock pulse CLK decreases the step-up ratio of the piezoelectrictransformer 11b. As a result, the driving circuit maintains the drivingelectric power constant. Thus, the frequency controlling circuit 11c iseffective against the fluctuation of the power voltage VDD.

The frequency controlling circuit 11c is a kind of signal processingcircuit, and is easily integrated on a semiconductor chip. The frequencycontrolling circuit 11c is conducive to the scale-down of the drivingcircuit and reduction of the production cost.

When the power voltage VDD is increased, the frequency controllingcircuit 11c makes the secondary currents Is1/Is2 of the electromagnetictransformers 11e/11f closer to the resonant waveforms due to thereduction of damping resulting from increase of impedance viewed fromthe primary electrodes 11i of the piezoelectric transformer 11b. Forthis reason, although the secondary currents Is1 and Is2 become zero attime t31, t32 and t33, the currents are changed across zero at earliertiming (see "secondary current Is1" in FIG. 14).

The current It is overlapped with the current Is2' between time t31 andtime t32, and the current Is2' partially cancels the peak of the currentIt. The current Is1 between time t32 and time t33 becomes negative. Evenif the gradient of the current It from time t33 is steep, the peakcurrent in the primary winding of the electromagnetic transformer is notwidely increased. Thus, the driving circuit according to the presentinvention keeps the peak current constant in spite of the undesirableincrease of the power voltage VDD, and an electromagnetic transformerwith a small capacitance is available for the auto-transformers 11e/11f.This is conducive to the reduction in volume.

As will be appreciated from the foregoing description, theauto-transformers 11e/11f achieve the same step-up ratio as the priorart electromagnetic transformers through a smaller number of turns ofthe winding, and make the driving circuit according to the presentinvention small and thin.

Especially, when the n-channel enhancement type switching transistor11g/11h turns on, the small inductance Lp1/Lp2 charges the primarywinding portion with a large amount of current from the power source. Onthe other hand, when the switching transistor 11g/11h turns off, thetotal inductance (Lp1+Ls1) or (Lp2+Ls2) makes the electrostaticcapacitance CL viewed from the input side of the piezoelectrictransformer 11b resonant. Thus, the primary winding portion is duallyused in the electromagnetic transformer 11e/11f, and theauto-transformers 11e/11f makes the driving circuit 11 small and thin.

Moreover, even if undesirable fluctuation takes place in the powervoltage, the frequency controlling circuit 11c and the 2-phase pulsegenerator 11d regulate the first and second pulse signals PL11/PL12 soas to appropriately drive the piezoelectric transformer 11b.

If the winding does not fill the bobbin of the electromagnetictransformer, a thick conductive wire is available, and increases theefficiency of the step-up circuit, because it decreases the loss of thetransformer.

The present inventor evaluated the driving circuit 11, and followingsare data obtained therefrom.

Piezoelectric transformer 11b

input capacitance Cd1: 2645 micro-F

output capacitance Cd2: 32.12 pF

resonant frequency: 111.3 KHz

Auto-transformer

primary inductance Lp: 60 micro-H

secondary inductance Ls: 214 micro-H

primary winding portion Tp: 25 turns

secondary winding portion Ts: 46 turns

turn ratio N: 1.84

winding on the primary side: 0.16 millimeter-phi UEW

winding on the secondary side: 0.12 millimeter-phi UEW

Cold Cathode fluorescent lamp

length L: 220 millimeters

diameter: 3 millimeters

Reflecting plate: yes

equivalent resistance RL2: 80 kilo-ohm

equivalent capacitance CL2: 10 pF

Driving Circuit

power voltage VDD: 8.0 volts

peak voltage of the primary voltage Vd1/Vd2: 29 V_(0-p)

peak voltage of the secondary voltage Vs1/Vs2: 82 V_(p-p)

step-up ratio Vs/Vd: 2.8

driving signal Vin: 433 Vrms=1220 V_(p-p)

discharge current Iout: 7.0 milli-A_(rms)

peak drain currents Id1/Id2: 0.56 A_(p-p)

peak primary currents Ip1/Ip2: 0.46 A_(p-p)

peak secondary currents Is1/Is2: 0.36 A_(p-p)

Although the particular embodiment of the present invention has beenshown and described, it will be obvious to those skilled in the art thatvarious changes and modifications may be made without departing from thespirit and scope of the present invention.

For example, the frequency controlling circuit 11c may regulates theclock signal CLK so as to regulate the driving signal Vin to a targetvalue. In this instance, the driving signal Vin is directly supplied tothe rectifier 11m as indicated by broken lines in FIG. 7, and thecurrent-to-voltage converter 11k is deleted from the frequencycontrolling circuit 11c.

The n-channel enhancement type switching transistors are replaceablewith p-channel enhancement type field effect transistors or bipolartransistors.

What is claimed is:
 1. A driving circuit for a load comprising:a firstauto-transformer including a first winding having a first node splittingsaid first winding into a first primary winding portion connected to apower source and a first secondary winding portion; a secondauto-transformer including a second winding having a second nodesplitting said second winding into a second primary winding portionconnected to said power source and a second secondary winding portion; afirst switching transistor including a first control node and a firstcurrent path connected between said first node and a constant voltagesource; a second switching transistor including a second control nodeand a second current path connected between said second node and saidconstant voltage source; a piezoelectric transformer including primaryelectrodes respectively connected to said first secondary windingportion and said second secondary winding portion, and a secondaryelectrode connected to said load; and a pulse generating means forsupplying a first pulse signal and a second pulse signal complementaryto said first pulse signal to said first control node and said secondcontrol node, respectively.
 2. The driving circuit as set forth in claim1, in which said pulse generating means includes a two-phase pulsegenerating circuit for generating said first pulse signal and saidsecond pulse signal from a clock signal, and a frequency controllingcircuit responsive to a discharge current from said load for regulatingsaid clock signal so as to make said piezoelectric transformer resonatearound a resonant frequency thereof.
 3. The driving circuit as set forthin claim 1, in which the secondary winding of one of said firstauto-transformer and said second auto-transformer supplies a secondarypotential to one of said primary electrodes of said piezoelectrictransformer, and said piezoelectric transformer supplies said secondarypotential from the other of said primary electrodes to the secondarywinding of the other auto-transformer and to said primary winding of theother auto-transformer, said secondary potential being larger than aprimary potential by a factor of one plus a turn ratio.
 4. The drivingcircuit as set forth in claim 1, in which said frequency regulatingcircuit includes a current-to-voltage converting means connected to saidload for converting said discharge current to a first potential signal,a first comparator coupled to said current-to-voltage converting meansso that said first comparator can change an output signal of saidcurrent-to-voltage converting means from a high level to a low levelwhen said first potential signal is higher than a first reference level,a resettable integrator connected to said first comparator andresponsive to said output signal of said current-to-voltage convertingmeans at said high level so that said resettable integrator cangradually decrease a second potential signal, a voltage-controlledoscillator connected to said resettable integrator and responsive tosaid second potential signal for proportionally changing a frequency ofsaid clock signal toward a target frequency close to a resonantfrequency of said piezoelectric transformer, and a second comparatorconnected to said resettable integrator and responsive to said secondpotential signal having a minimum potential level so as to supply areset signal to said resettable integrator.